I. Introduction of Implementation of Fax Services in a Packet-Based Network.
Information technologies will keep rapid development in the 21st century, and human beings have entered an age of network economy. With the rapid development of network technologies, transmission of multimedia using packet technologies becomes possible and even comes true. Because media stream coding manners are different in different communication networks, it is necessary to use a codec to convert the media stream coding manners at joints of different communication networks. A device which performs the conversion is called a “gateway”.
Media streams now processed by the gateway mainly include voice streams, data streams, video streams, etc. The data streams mainly refer to signals transmitted by a data device such as a fax, a modem, or a text telephone during an interaction process. Currently, main manners for transferring data streams through a gateway in the industry include: VBD manner and Relay manner.
The VBD manner refers to a coding/decoding manner with less signal impairment, which treats data streams as normal voice streams and performs a low loss coding/decoding process. The proposed coding/decoding standards are ITU-T G.711 and ITU-T G.726. The advantage of VBD manner is that it is easy to implement and it does not need to be concerned about the concrete meaning of the data signals and the data signals simply need to be treated as voice signals. In this way, the VBD manner consumes very little of the processing capability of the gateway. The disadvantage of VBD manner is that this manner occupies a large amount of bandwidth, easily subjected to fluctuations of Internet Protocol (IP) networks, and vulnerable to packet loss and delay occurred in IP networks. The VBD manner may be applicable in processing data streams of various data devices.
The Relay manner refers to a manner that coverts by demodulation, according to some protocols that specify data signal conversion and transmission method, data signals sent by a sender data device, to data messages that are adapted to be transferred over an IP bearer network, transfers the data messages to a remote gateway, and then recovers by modulation, the data signals and sends the data signals to a receiver data device. Examples of Relay manners include Fax Relay in ITU-T T.38, Modem Relay in ITU-T V.150.1. The advantage of the Relay manner is that it is robust to various impairments that may occur in the IP network, and may considerably increase the possibility of success and stability of data services. The disadvantage of the Relay manner is that it is relatively complex and consumes a large amount of processing capability of the gateway.
With the constant development of data services, more and more types of data services appear. The VBD manner draws more and more attention due to its easy implementation and wide application. Further, some application optimizations have been done for VBD manner, such as to implement redundant process of Real-time Transport Protocol (RTP) messages according to the IETF RFC2198 protocol, to implement a static Jitter Buffer at a message receiver. The capability of resistance to packet loss and delay over IP networks of VBD manner is considerable enhanced.
However, the problem that VBD manner occupies a large amount of bandwidth is not solved well. Taking the coding/decoding manner specified by G.711 protocol as an example, the bandwidth occupied by data payload is 64 k bit/s when RFC2198 redundancy is not applied. The occupied bandwidth is 128 k bit/s when 1 frame is redundant. The occupied bandwidth is 192 k bit/s when 2 frames are redundant. Such requirement is not acceptable in some applications with limited bandwidth.
II. Introduction of Function of Data Signal Detector.
In conventional VBD manner, all of the data signals sent by a data device are treated as voice signals and are inputted into a voice encoder for a low loss coding/decoding process. FIG. 1 illustrates a conceptual diagram of a communication system in conventional VBD manner. As shown in FIG. 1, after data signals pass a voice encoder, a sender gateway sends the processed signals to a receiver gateway via a communication channel, and the receiver gateway outputs reconstructed data signals via a voice decoder.
However, in processes of many data services, it is very often that the data device only sends mute signals, not effective data signals. Taking the widely used fax machine, which supports a highest rate of 14400 bit/s as an example, its fax process is half duplex, that is, the fax machine sends mute signals in more than 50% of the time of the fax process.
In consideration of the above situation, it is proposed that a data signal detector be used in a system utilizing VBD manner, so that the effective data signals and mute signals sent by the data device may be distinguished and inputted into different encoders for coding. FIG. 2 illustrates a conceptual diagram of a communication system in a VBD manner using a data signal detector. The concept of the system is shown in FIG. 2.
The system includes three modules: a sender gateway, a communication channel, and a receiver gateway. The sender gateway includes a data signal detector, a low loss voice encoder and a non-voice encoder. The receiver gateway includes a voice decoder and a non-voice decoder. The communication channel is generally an IP network.
When data signals enter the sender gateway, the data signals are firstly divided into signal frames with equal intervals. The length of the frame is determined according to a coding protocol used by the encoder. Generally, the length is between 5-30 ms. Then the data signal detector analyzes each inputted signal frame and categorizes the frames into data signal frames and non-data signal frames based on whether the frame carries effective data signals or not. If the inputted signal frame is a data signal frame, the data signal detector controls the signal frame to enter the voice encoder for low loss voice coding; otherwise, the data signal detector controls the signal frame to enter the non-voice encoder for mute signal compression coding. Information outputted by the voice encoder is called a voice packet, while information outputted by the non-voice encoder is called a mute packet. Information in the mute packets is only used to restore mute signals. Therefore, the rate of mute packets is very low, less than 1/10 of the rate of voice packets.
When the receiver gateway receives the voice packets, the receiver gateway inputs the voice packets into a voice decoder for voice decoding and outputs the effective data signals. When the receiver gateway receives the mute packets, the receiver gateway inputs the mute packets into the non-voice decoder for reconstruction of the mute signals.
When using the data signal detector to categorize the signal frames into data signal frames and non-data signal (mute) frames, and handling these frames in different coding manners, only a little mute information is sent during a non-effective data signal period. Compared with the situation that all of the signal frames are inputted into the voice encoder without distinction, such a manner reduces an output code rate considerably.
III. Introduction of Control Protocol
The H.248 protocol is specified by International Telecommunications Union—Telecommunication Standardization Sector (ITU-T), and is specialized for media resource control. The H.248 protocol may be applied to control media processing devices such as media gateways, media servers. The basic concept of the H.248 protocol is that all of the resources on the media processing devices are abstracted as terminations which are further categorized into physical terminations and temporary terminations. The physical terminations represent some semi-permanent physical entities, such as Time Division Multiplex (TDM) timeslots. The temporary terminations represent public resources that are temporarily requested for use and released after usage, such as RTP streams. Combination of the terminations is abstracted as a context, which describes relationship between the terminations by topology. Based the abstracts, call connections are actually operations on the terminations and the context, and are accomplished by command requests and responses between media control devices and media processing devices. Commands include addition, modification, deletion, notification, etc. Command parameters are called descriptors, which are categorized into Property, Signal, Event and Statistic.
The Session Initiation Protocol (SIP) is specified by Internet Engineering Task Force (IETF), and is specialized for multimedia communication. According to the definition of the IETF RFC3261 protocol, SIP is a text based application layer control protocol, and is independent of an underlying layer transport protocol. SIP is adapted to establish, modify or terminate a two-party or multi-party multimedia session on an IP network. SIP protocol supports the functions such as proxy, redirection and user registration and location. By cooperating with associated protocols, SIP may also support functions such as voice, video, data, email, chat and game.
The Session Description Protocol (SDP) is specified by IETF, and is specialized for media stream information transmission in media sessions. SDP defines a uniform format of session description, including the following aspects:
1) name and object of a session;
2) life time of a session;
3) media information involved in a session, including: media type (such as video, voice etc.), transport protocol (such as RTP/UDP/IP, H.320 etc.), media format (such as H.263, G.711 etc.), address and port for multicast or far-end (unicast);
4) information needed for receiving the media (such as address, port, format etc.);
5) used bandwidth information; and
6) trusted Contact information.
Protocols such as SIP, H.248 all use SDP to describe properties of media resource.
Because the data signal detector is applied in a VBD mode, an extension of SDP during VBD capability negotiation in prior art is described below.
Under ITU-T V.152 protocol, when negotiating VBD capability, it is necessary to extent a “gpmd” (general-purpose media descriptor) property with a “vbd=yes” field, for example:m=audio 3456 RTP/AVP 0 18 98a=rtpmap:98 PCMA/8000a=gpmd:98 vbd=yes
The 0, 18, 98 in the first line are payload types of RTP packets. According to IETF RFC3551 protocol, a payload type lower than 96 is a static payload type. The meaning of static payload type is fixed and is specified in RFC3551. A payload type between 96 and 127 is a dynamic payload type. The meaning of dynamic payload type is defined temporarily during usage. In the above embodiment, the first line means that the device supports three payload types 0, 18 and 98, where 0 represents G.711 μlaw, 18 represents G.729. The second line defines that a dynamic payload type 98 represents G.711 A law with a sample rate of 8000 Hz. The third line represents that the payload type 98 is applied to VBD manner, which implies that VBD is supported.
The deficiency of the prior art is that data signal detector capability negotiation cannot be implemented.